The present invention relates to a voltage comparator circuit for comparing a signal voltage with a reference voltage.
A voltage comparator circuit generally comprises an amplifer having a large gain. The voltage comparator cicuit is often used to compare magnitudes of various signal voltages and to change the operation and/or state of the system, device, or circuit in accordance with the comparison result. When an accurate comparison is required, the precision, i.e., the magnitude of the offset voltage of the voltage comparator circuit itself is important. When a voltage comparator circuit is incorporated in an integrated circuit, the magnitude of the offset voltage cannot be generally obtained in advance. More specifically, the offset voltage changes in accordance with the surrounding atmosphere (e.g., the ambient temperature) and/or the time lapse, and this change cannot be predicted. In addition, such an offset voltage cannot often be measured from outside. If such a voltage comparator circuit is used, the comparison determination level has uncertainty corresponding to the offset voltage. Therefore, in design of a voltage comparator circuit, the offset voltage must be suppressed as low as possible. Particularly, when an active element constituting a voltage comparator circuit is a MOSFET, the offset voltage is larger than a case wherein the active element comprises a bipolar transistor. In this case, the problem caused by the offset becomes serious.
Conventionally, several techniques for compensating the offset of an amplifier are proposed in order to solve the above problem. In these techniques, the offset voltage is charged in a capacitor by utilizing a very high input impedance of a MOSFET, and the offset of the amplifier is compensated using the charged capacitor voltage.
For example, Poujois et al. published "A LOW DRIFT FULLY INTEGRATED MOSFET OPERATIONAL AMPLIFIER" (IEEE JOURNAL OF SOLID-STATE CIRCUIT, VOL. SC-13, No. 4, pp. 499-503, AUGUST 1978). FIGS. 1A and 1B are views for explaining the principle of the amplifier by Poujois et al.
FIG. 1A shows a voltage comparator circuit having an offset. In FIG. 1A, the offset voltage is extracted from the voltage comparator circuit for the sake of descriptive convenience. In this case, the voltage comparator circuit consists of differential voltage comparator 201 having no offset and voltage source 202 representing the offset voltage (originally included in comparator 201) extracted from comparator 201. FIG. 1A also shows positive phase input terminal 203 to which a voltage to be compared is applied, negative phase input terminal 204 to which a voltage as a comparative reference is applied, output terminals 205 and 206 of comparator 201, and positive phase input terminal 200 of comparator 201. The voltage comparator circuit shown in FIG. 1A has an offset. Therefore, when a voltage higher than reference voltage 204 by the offset voltage 202 or more is applied to input terminal 203, the voltage at output terminal 205 (206) becomes negative (positive) and has a level corresponding to the magnitude of the voltage applied to terminal 203. Inversely, when a voltage lower than reference voltage 204 is applied, the voltage at terminal 205 (206) becomes positive (negative) and has a level corresponding to the magnitude of the voltage applied to terminal 203. In short, an error in voltage comparison corresponding to offset voltage 202 occurs.
A circuit shown in FIG. 1B is used in order to compensate for the offset voltage. A voltage comparator circuit shown in FIG. 1B has switches 207 to 214 controlled from outside, and capacitors 215 and 216 to be charged with the offset voltage. In FIG. 1B, the circuit portion within a broken line corresponds to the voltage comparator circuit shown in FIG. 1A. FIG. 1B also shows input terminals 217 and 218 to which a voltage to be compared and a voltage as a comparison reference are applied, and output terminals 219 and 220 at which a signal corresponding to the comparison result is output.
Referring to FIG. 1B, initially switches 209 to 212 are closed and switches 207, 208, 213, and 214 are opened. One end of each of switches 209 to 212 is connected to an analog ground potential (to be indicated as V.sub.AG). In this state, a voltage determined by offset voltage 202 appears at output terminals (205, 206) of voltage comparator circuit 201 and is charged in capacitor 215 and 216. Subsequently, switches 209 to 212 are opened and switches 207, 208, 213, and 214 are closed. Then, the voltage at output terminals (205, 206) of voltage comparator 201 takes a value determined by (voltage at input terminal 217) --(offset voltage 202)--(voltage at input terminal 218) . However, since capacitors 215 and 216 are already charged with a voltage determined by offset voltage 202, the voltages at terminals 219 and 220 are determined solely by (voltage at terminal 217)--(voltage at terminal 218) , and apparent offset voltage 202 is compensated. Therefore, when the switches are cyclically controlled to repeat these states, even if the offset of comparator 201 fluctuates, the offset is periodically compensated at the switching cycle of the switches, and a highly precise voltage comparison result can be obtained. In this example, the offset is compensated at the output side. Therefore, the circuit must be designed such that the gain of the voltage comparator may not become excessive, thus preventing the output voltage from being saturated by the offset voltage.
Voltage comparators using MOSFETs are often operated by a single power source voltage (V.sub.DD). In this case, analog ground potential V.sub.AG is generally selected to be V.sub.DD /2 in consideration of the dynamic range of the circuit. However, the reference voltage of the voltage comparison is often a voltage (V.sub.AG +.DELTA. V or V.sub.AG -.DELTA.V) which is deviated from potential V.sub.AG by .DELTA.V, and in some cases .DELTA.V is externally supplied with reference to the power source of 0 V as a reference. In this case, voltage .DELTA.V externally supplied is level-shifted to V.sub.AG +.DELTA.V or V.sub.AG -.DELTA.V in order to generate a reference voltage of the voltage comparator. However, if the circuit for the level shift has an offset, an accurate comparison result cannot be obtained no matter how correct the voltage comparator itself may be. Therefore, an offset compensating operational amplifier is conventionally used as a level shifter. For example, IEEE Journal of Solid-State Circuits, T. Habuka et al. Vol. SC-20, No. 2, April 1985, p. 620, FIG. 4 shows a "REFERENCE CONVERTER" for level-shifting .DELTA.V to V.sub.AG +.DELTA.V and VAG-.DELTA.V. An output from the "REFERENCE CONVERTER" is compared with a signal "IN" and an accurate comparison result can be obtained.
However, since a reference voltage generator, i.e., a level shifter must be provided in order to constitute a voltage comparator, the size of the entire voltage comparator becomes large. In addition, when the circuit size becomes large, the reliability may be degraded, resulting in disadvantage in terms of economy.
As described above, in the conventional voltage comparator circuit, in order to generate a reference voltage of a voltage comparator, a special level shifter must be provided independently of the voltage comparator.